1. Field of the Invention
The present invention relates to a gas concentration detection apparatus which detects the concentration of a specific gas component in a gas of interest by measuring a current which flows through a gas sensor and whose magnitude changes in accordance with the concentration of the specific gas component. The present invention also relates to a gas concentration detection system which combines the gas sensor and the gas concentration detection apparatus.
2. Description of the Related Art
Conventionally, a gas sensor has been known which is installed in an exhaust passage of an internal combustion engine, such as an automotive engine, and which detects the concentration of a specific gas component (for example, oxygen) in exhaust gas. This gas sensor detects the oxygen concentration and ultimately the air fuel ratio of the exhaust gas, by making use of a phenomenon in which the magnitude of a current flowing through a sensor element changes in accordance with the oxygen concentration of the exhaust gas. In a gas concentration detection apparatus (for example, an ECU (electronic control unit)) connected to the gas sensor, a current detection resistor is connected to the gas sensor so that a current output from the gas sensor flows through the detection resistor. By measuring the potential difference generated across the detection resistor, the oxygen concentration of the exhaust gas is obtained, and the air fuel ratio of the exhaust gas is obtained based on the oxygen concentration. Then, the thus-obtained oxygen concentration or air fuel ratio of the exhaust gas is utilized for air fuel ratio feedback control, which is performed, for example, for adjusting the quantity of fuel injected into the engine.
In recent years, regulation of automotive exhaust gas has become more stringent. In order to meet stricter regulations, there is a need for improved accuracy in performing feedback control of the air fuel ratio near the stoichiometric air fuel ratio (in a stoichiometric range), and in performing feedback control of the air fuel ratio within a predetermined lean range. In a general gas concentration detection apparatus, a voltage output from a detection resistor is converted to digital data, which in turn is input to a CPU so as to perform air fuel ratio feedback control under software control. In general, the gas concentration detection apparatus is configured such that an entire range within which the air fuel ratio varies corresponds to an entire voltage range within which the output voltage of the detection resistor can be accommodated for digital conversion. Therefore, a voltage range assigned to (corresponding to) the stoichiometric range is narrow as compared with the entire voltage range within which the output voltage of the detection resistor can be input for digital conversion. In a known technique, when the output voltage of the detection resistor enters the stoichiometric range, the output voltage of the detection resistor is amplified such that the voltage range assigned to the stoichiometric range is expanded to the entire voltage range within which the output voltage can be accommodated for digital conversion, to thereby improve the detection accuracy of the air fuel ratio in the stoichiometric range (refer to, for example, Patent Document 1).    [Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2006-275628
3. Problems to be Solved by the Invention
However, when a circuit which acquires and amplifies the output voltage (potential difference) of the detection resistor is affected by variations in characteristics of electronic components of the circuit and the temperature characteristics thereof, the detected air fuel ratio may contain an error. Especially, when the output voltage of the detection resistor is amplified in the stoichiometric range, the error is also amplified. Accordingly, there is a need to improve accuracy in detecting the air fuel ratio.